Conventional golf balls can be divided into two general classes: solid and wound. Solid golf balls include one-piece, two-piece (i.e., single layer core and single layer cover), and multi-layer (i.e., solid core of one or more layers and/or a cover of one or more layers) golf balls. Wound golf balls typically include a solid, hollow, or fluid-filled center, surrounded by a tensioned elastomeric material, and a cover.
Golf ball core and cover layers are typically constructed with polymer compositions including, for example, polybutadiene rubber, polyurethanes, polyamides, ionomers, and blends thereof. Ionomers, particularly ethylene-based ionomers, are a preferred group of polymers for golf ball layers because of their toughness, durability, and wide range of hardness values.
Golf ball compositions comprising highly neutralized acid polymers are known. For example, U.S. Patent Application Publication No. 2003/0130434, the entire disclosure of which is hereby incorporated herein by reference, discloses melt-processible, highly-neutralized ethylene acid copolymers and process for making them by incorporating an aliphatic, mono-functional organic acid in the acid copolymer and then neutralizing greater than 90% of all the acid groups present. The use of such compositions in various golf ball layers is disclosed. Also, U.S. Patent Application Publication No. 2005/0148725, the entire disclosure of which is hereby incorporated herein by reference, discloses a highly-resilient thermoplastic composition comprising (a) an acid copolymer, (b) a salt of a high molecular weight, monomeric organic acid; (c) a thermoplastic resin; (d) a cation source; and (e) optionally, a filler. The reference also discloses one-piece, two-piece, three-piece, and multi-layered golf balls comprising the highly-resilient thermoplastic composition.
While various uses for highly neutralized acid polymers in golf balls have been discovered, there is a need in the industry to broaden the applicability of highly neutralized acid polymers to particular golf ball constructions having desirable spin, feel, and COR properties. The present invention provides such golf ball constructions through the use of a layer formed from a relatively soft (or relatively low modulus) HNP composition and a layer formed from a relatively hard (or relatively high modulus) HNP composition.